Ultra-thin MFI membranes for olefin/nitrogen separation
Introduction
Vapour-gas separation systems emerged in petrochemical and refinery industries due to the high economic value of the recovered hydrocarbons. Recovery of light hydrocarbons from petrochemical process streams, for example the purge gas from a polypropylene or polyethylene plant resin degasser, is an interesting application. Typically, the purge stream from a polypropylene plant contains 20–30% C3+ hydrocarbons in nitrogen, and it would be of great value for the industry to recycle this part and use the monomers for polymer production instead of burning it [1], [3], [4].
Membrane separations are particularly appealing for gas purification due to their low energy consumption, good selectivity and low costs. From organic to inorganic, various materials of membranes have been evaluated for separation of organic vapour/gas mixtures. Silicone rubber is by far the most widely used membrane material for recovering hydrocarbons from vent streams, as summarized by Baker et al. [1]. Silicon rubber membranes are well-suited for separation of organic vapour/gas mixtures and the commercialization of this type of membrane continue to stimulate membrane invention and industrial demand. However, these membranes are quite thick (typically ca. 50–100 µm), which partially explains why these membranes show rather low permeances. Furthermore, the selectivity is only 8–12 for propylene/nitrogen and ethylene/nitrogen mixtures [1,5]. Consequently, large membrane area and many modules are needed, which increases the capital cost of the facility. Besides, the membranes show rather low stability, freshly made thin composite polymer membranes will often lose 50% of the permeance within two weeks in this application. Paul et al. have shown that the performance deterioration results from the reordering of the polymer chains in the membrane, thus reducing the number and size of free volume elements that contribute to gas permeation [6]. Therefore, a key challenge for polymer membranes is to increase the long term stability. Enhanced performance may be obtained by combinations of polymers with inorganic particles in mixed-matrix membranes, but difficulties, for example achieving good dispersion [7], are frequently encountered for this type of membranes.
Over the past decades, the development of inorganic membranes, especially zeolite membranes, has gained increased interest due to the potentially high stability, permeability and selectivity. MFI zeolite is one of the most explored zeolites for membrane applications. MFI zeolite has a well-defined system of pores with a size of ca. 0.55 nm in diameter. Furthermore, the Si/Al ratio of this zeolite can be varied considerably making it possible to tailor the properties to a large extent, for instance hydrophobicity and ion-exchange capacity, which could open up for using of the membranes in a broad range of liquid and gas mixture separation applications [8], [9]. However, to the best of our knowledge, no olefin/nitrogen separation by MFI membrane has been reported until now. Thus far, additionally, most reports on zeolite membranes described relatively thick zeolite films; consequently permeances are typically quite low, comparable to polymeric membranes. However, to be competitive, zeolite membranes must show much higher permeance than polymeric membranes [10].
Our group has developed a technique for preparing ultra-thin (ca. 500 nm) MFI zeolite membranes on open graded supports showing very high permeances e.g. a CO2 permeance of 93×10−7 mol m−2 s−1 Pa−1 for 50/50 CO2/H2 binary mixture with a separation factor of 16.2 [11]. In addition, very high fluxes have been reported for p-xylene [12] and also for ethanol and n-butanol [13], [14].
In the present work, we evaluated our ultra-thin MFI membranes for separation of propylene/nitrogen and ethylene/nitrogen mixtures (20 mol% olefins in nitrogen) for the first time. The separations were performed at different temperatures to identify the optimum separation conditions. The effect of concentration polarization and pressure drop over the support were estimated by modelling.
Section snippets
Membrane preparation and characterization
The membranes were prepared using a seeding method on masked supports as described in detail previously [15]. Porous graded α-alumina discs (Fraunhofer IKTS, Germany) with a diameter of 25 mm comprised of a 30 µm thick top layer with a pore size of 100 nm and a 3 mm thick base layer with a pore size of 3 µm were used as supports. The supports were masked with polymethylmethacrylate (PMMA, Mw=100 000 g mol−1, q=1.8, CM 205, Polykemi AB, Sweden) and a Fisher Tropsh wax (Sasolwax C105, Carbona AB,
Membrane characterization
Top view and cross-sectional SEM images of an as-synthesised MFI membrane are shown in Fig. 1. The top view shows that the film is continuous film and is comprised of well intergrown zeolite crystals with a maximum size of about 400 nm at the top surface of the membrane. No defects could be observed by SEM in the membranes. The cross-sectional image shows that the film appears to be rather even with a total thickness including invasion of around 420 nm. The XRD data in Fig. 1 shows that the film
Conclusions
Ultra-thin (0.5 µm) MFI zeolite membranes were used to separate propylene or ethylene from binary mixtures with nitrogen at different temperatures. The membrane showed a high performance for both separations. The highest propylene/nitrogen separation factor and separation selectivity were 43 and 80, respectively, coupled with a permeance of 22×10−7 mol m−2 s−1 Pa−1 at room temperature. The maximum separation factor and separation selectivity for ethylene/nitrogen gas mixture were 6 and 8.4,
Acknowledgements
The Swedish Energy Agency (Grant no. 38336-1) are gratefully acknowledged for financially supporting this work.
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